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Acoustics studies sound: its generation, propagation, interaction with matter and detection. In a fluid, a small acoustic wave couples a pressure disturbance to the motion of the medium. The disturbance transports energy without requiring the same parcel of material to travel from the source to the receiver.

A loudspeaker and a microphone make a simple experiment. Changing their separation measures propagation delay, while changing the frequency in a bounded room reveals resonance and standing waves. A strong microphone response does not by itself imply a large net sound intensity: oppositely travelling waves can produce a large local pressure while their mean energy transport cancels. Acoustic impedance connects pressure and motion for a specified travelling wave.

An acoustic transducer converts between an acoustic disturbance and another form of signal or actuation. A microphone converts sound-induced motion into an electrical signal; a loudspeaker performs the opposite conversion. A piezoelectric transducer can use reciprocal coupling between deformation and an electric field to transmit and receive ultrasound.

Its frequency response, sensitivity and acoustic impedance affect what is emitted or measured. In a pulse-echo experiment, the same device can launch a short pulse and record the returning reflection. Travel time constrains distance only after the sound speed and the outgoing-plus-return path are accounted for.

Electroacoustics studies conversion between electrical signals and sound, together with the measurement and control of that conversion. A microphone produces an electrical response to an acoustic input; a loudspeaker performs the reverse conversion. Their frequency responses, loading and directional behavior matter as much as a single sensitivity number.

For a receiving microphone, one can define the complex frequency response $M_p(f)=V(f)/p(f)$ in volts per pascal, with the electrical termination and acoustic conditions specified. Equal outputs do not imply equal incident sound pressures if the two microphones have different sensitivities. Calibration supplies the link between the electrical recording and an acoustic measurement.

A motivating primary method is microphone reciprocity calibration. In the NIST system reported by Randall P. Wagner and collaborators, three laboratory microphones are measured in pairs using acoustic coupling cavities. One microphone transmits while another receives; known coupling properties and reciprocity relations allow the individual pressure sensitivities to be determined without starting from a separately calibrated reference microphone. Comparing results with different cavity lengths tests the acoustic corrections and associated measurement uncertainty.^[ref]

An electroacoustic transducer converts between an electrical signal and acoustic motion. A loudspeaker or a pair of headphones converts electrical input into sound; a microphone converts acoustic input into an electrical output. The name describes the conversion role, not one universal physical mechanism.

In an idealized moving-coil device, a current-carrying conductor in a magnetic field experiences a force. If the active conductor length is $\ell$ and the relevant field strength is $B$, the perpendicular-component model gives $F=B\ell I$. With $B\ell =2N/A$ and current $I=0.020\mathrm{A}$, the force is $0.040\mathrm{N}$. A diaphragm and its mechanical load turn that force into motion and sound pressure; force alone does not determine the complete acoustic output.

Conversion therefore has several linked stages: electrical drive, mechanical motion and acoustic coupling. A pressure transfer function of the complete device can depend on frequency and its surroundings. Headphone measurements on an acoustic mannequin, for example, varied when the cushions were repositioned, despite using the same nominal transducer. This demonstrates a change in the coupled system rather than requiring a change in the electrical recording itself.^[ref]

The distinction is useful in calibration. A quoted electrical sensitivity, mechanical displacement or acoustic pressure is meaningful only with its input, measurement point and loading conditions specified. An electroacoustic transducer transfers energy and information through a physical interface; its useful behavior belongs to the device together with that interface.

PHYWE's implementation rubs a $1.60\mathrm{m}$ rod supported $0.40\mathrm{m}$ from each end. Its end disc drives a dusty glass tube; a movable tuning slide selects a clear pattern. A ruler measures repeated positions and a thermometer records gas temperature. The two supports select a rod wavelength equal to its length.^[ref]

The University of Iowa also documents an electronic version: a loudspeaker drives the tube and a microphone on a plunger supplies an oscilloscope readout. This permits a spatial scan of sound pressure without relying on the dust pattern.^[ref]

For a proposed pressure-scan measurement, keep the drive frequency fixed and move a small microphone through the tube. Record its position and fitted sinusoidal amplitude after each move; repeat the scan in reverse to reveal drift. Locate several consecutive maxima, rather than estimating a wavelength from a single pair. Reducing the drive checks whether the inferred spacing is independent of amplitude in the linear acoustics regime. A drive-off record estimates electrical background. These are controls for the proposed measurement, not reported observations from the cited demonstrations.

A microphone and its access opening can perturb the field. End compliance, losses and transverse modes also challenge the one-dimensional description. Dust traces identify a repeating structure, but the grains are not fluid parcels: the equations below describe sound pressure and acoustic particle velocity, rather than a detailed grain-transport model. The temperature and gas composition must remain sufficiently uniform to interpret a single fitted speed of sound.

A loudspeaker converts an electrical signal into motion that drives acoustic waves. In a moving-coil design, electric current through a coil in a magnetic field produces a force on the attached diaphragm. The diaphragm displaces air, creating sound pressure variations.

A small coil, magnet and lightweight cup can demonstrate the conversion directly. ^[ref] In an idealized uniform-field geometry, the driving force scales as $B\ell I$. The emitted sound also depends on diaphragm inertia, suspension spring constant, damping and acoustic impedance, so the electrical waveform alone does not specify a flat acoustic frequency response.

Use a plane wave proportional to $e^{i(kx-\omega t)}$. Substitution into the convected acoustic wave equation gives the dispersion relationThe two branches have signed phase velocities and group velocities $U_0\pm c_0$. On a branch with $\omega -U_{0}k=s{c}_{0}k$, $s=\pm 1$, the linearized Euler equation givesIn subsonic flow with $0\le U_0<c_0$, the absence of wave reflection selects the outgoing branch on each side:For $U_0=0$, take $A$ real and positive by choosing the source phase, and put $k_0=\omega /c_0$. At $\omega t=2n\pi$, the requested spatial profiles areFor complex $A$, retain $\mathrm{Re}[A{e}^{i{k}_0|x|}]$ instead. The normalized sound pressure and acoustic density perturbation lie on the same even cosine curve. The acoustic particle velocity follows that curve on the right and its negative on the left, as in the figure in part 2(a). Compressed fluid moves away from the source on both sides. These are two outgoing travelling waves, not a standing wave. The ideal point loudspeaker supplies the discontinuity of velocity at $x=0$; the source-free equations apply on either side of it.